compare a large number of variables

[Einar]

Hi,

I wonder if there is a nice bit twiddling hack to compare a large
number of variables?

If you first store them in an array, you can do:

for (i = 0; i < n; i++) {
if (array != value) {
/* array differs from value, do something*/
}
}

but I dont have the variables in an array, and would like to figure out
a nice oneliner.

best regards.
E.

 

[Peteris Krumins]

Hi,

I wonder if there is a nice bit twiddling hack to compare a large
number of variables?

If you first store them in an array, you can do:

for (i = 0; i < n; i++) {
if (array != value) {
/* array differs from value, do something*/
}
}

but I dont have the variables in an array, and would like to figure out
a nice oneliner.

N nice one liners:

if (var1 != value) { /* do smth */ }
if (var2 != value) { /* do smth */ }
....
if (varN != value) { /* do smth */ }


P.Krumins

 

[Eric Sosman]

Hi,

I wonder if there is a nice bit twiddling hack to compare a large
number of variables?

If you first store them in an array, you can do:

for (i = 0; i < n; i++) {
if (array != value) {
/* array differs from value, do something*/
}
}

but I dont have the variables in an array, and would like to figure out
a nice oneliner.

I'm not sure what you're looking for, but it might
be one of

if (a != value || b != value || ... || z != value) {
/* at least one of a,b,...,z differs from
* value, do something */
}

or

if (a != value && b != value && ... &&& z != value) {
/* all of a,b,...,z differ from value, do
* something */
}

or

if (a != value) {
/* a differs from value, do something */
}
if (b != value) {
/* b differs from value, do something */
}
...
if (z != value) {
/* z differs from value, do something */
}

or as above but with `else' before all but the first `if'

or

int *ptr[] = { &a, &b, ..., &z };
for (i = 0; i < n; ++i) {
if (*ptr != value) {
/* The i'th of a,b,...,z differs from
* value, do something */
}
}

If none of these is what you're trying to do, you'll
have to explain your intent more clearly.

 

[Peteris Krumins]

Hi,

I wonder if there is a nice bit twiddling hack to compare a large
number of variables?

If you first store them in an array, you can do:

for (i = 0; i < n; i++) {
if (array != value) {
/* array differs from value, do something*/
}
}

but I dont have the variables in an array, and would like to figure out
a nice oneliner.

N nice one liners:

if (var1 != value) { /* do smth */ }
if (var2 != value) { /* do smth */ }
....
if (varN != value) { /* do smth */ }


P.Krumins

 

[Keith Thompson]

I wonder if there is a nice bit twiddling hack to compare a large
number of variables?

If you first store them in an array, you can do:

for (i = 0; i < n; i++) {
if (array != value) {
/* array differs from value, do something*/
}
}

but I dont have the variables in an array, and would like to figure out
a nice oneliner.

Perhaps they should be in an array.

If that's not an option, and you're going to be doing this a lot, you
might consider setting up an array of pointers to the variables and
looping over that.

 

[Jack Klein]

Hi,

I wonder if there is a nice bit twiddling hack to compare a large
number of variables?

If you first store them in an array, you can do:

for (i = 0; i < n; i++) {
if (array != value) {
/* array differs from value, do something*/
}
}

but I dont have the variables in an array, and would like to figure out
a nice oneliner.

best regards.
E.

Here is a "trick" that I do not really recommend as best practice, but
I have used in small microcontrollers for embedded systems with severe
memory constraints.

Assuming you have a header that contains the following:

my_vars.h

extern int val_a;
extern int val_b;
/* ... */
extern int val_y;
extern int val_z;

....and of course you have a source file:

my_vars.c

int val_a;
int val_b;
/* ... */
int val_y;
int val_z;

....then you could change my_vars.h to:

extern int sneaky_array[];
#define val_a sneaky_array[0]
#define val_b sneaky_array[1]
/* ... */
#define val_y sneaky_array[24]
#define val_z sneaky_array[25]

....and change my_vars.c:

int sneaky_array[2];

....then you can write new code that can loop through the values as an
array, whereas older source code that refers to individual elements by
name will still compile and work with the new my_vars.h header.

But I really, really don't recommend this except for exceptional
circumstances.

 

[Einar]

Hello Eric,

Yes, your suggestions work perfectly, but I was mor looking for a nice
bit operator to operate on all my variables and then to compare it with
the value. something like (a|b|c|d...z) != value (this wont work
however... ). This is just to get rid of a for-loop or lots of
if-statements, so it is nothing necesary, I'm just convinced that it is
possible to solve in another way, and I can't forget about it....
Ahhhrg.

best regards.
E.

 

[AM]

Hi,

This might help you...

/* file.e */
USE(a)
USE(b)
USE(c)

/* file.c */
/* Inside your function */
#define USE(var) do { \
if ( ##var != value) \
{ \
/* your stuff */ \
} \
while(0);
#include "file.e"

you can compile the code with -E option (gcc) and see the expanded code
as follows-

do { if (a > value ) { } while (0);
do { if (b > value ) { } while (0);
do { if (c > value ) { } while (0);

Regards,
-A M

 

[Netocrat]

Hello Eric,

Yes, your suggestions work perfectly, but I was mor looking for a nice
bit operator to operate on all my variables and then to compare it with
the value. something like (a|b|c|d...z) != value (this wont work
however... ). This is just to get rid of a for-loop or lots of
if-statements, so it is nothing necesary, I'm just convinced that it is
possible to solve in another way, and I can't forget about it....
Ahhhrg.

How about (off the top of my head and untested):

if ( !( ((a|b|c|d...z) == value) && ((a&b&c&d...z) == value) ) )
/* one or more of a,b,c,d...z is not equal to value */

I don't think that would generally be as efficient as using multiple
comparisons against value because in that case the first mismatch will
prevent the rest from being evaluated. You never know though - on some
hardware it might be; or the compiler might rewrite it for you.

Another alternative (also untested):

if ( ((a|b|c|d...z)&a&b&c&d...z) != value )
/* one or more of a,b,c,d...z is not equal to value */

 

[freak]

Rerwite code to have them all grouped in a user defined type and then
use memcmp().

 

[Flash Gordon]

Rerwite code to have them all grouped in a user defined type and then
use memcmp().

All what? Learn to quote. The google interface may by broken but it is
possible, and if you had read this (or almost any other group) you would
have seen that it is the done thing.

Search this group for broken google interface and you will see *lots* of
complaints, explanations of the problem, and instructions on how to get
Google to do the right thing.

 

[Peter Pichler]

#define USE(var) do { \
if ( ##var != value) \
{ \
/* your stuff */ \
} \
while(0); ....
you can compile the code with -E option (gcc) and see the expanded code
as follows-

do { if (a > value ) { } while (0);
do { if (b > value ) { } while (0);
do { if (c > value ) { } while (0);

That's right. You can see the expanded code. You can't compile it
though. (Missing right brace.)

Peter

 

[Michael Wojcik]

How about (off the top of my head and untested):

if ( !( ((a|b|c|d...z) == value) && ((a&b&c&d...z) == value) ) )
/* one or more of a,b,c,d...z is not equal to value */

Or the similarly silly:

if (a^value|b^value|c^value|...|z^value)
/* one or more of a,b,c,...,z is not equal to value */

(Untested, but I think that's right.) Or similarly:

if (a^b|b^c|c^d|...|y^z|z^value)

All assuming that we're working with values that aren't going to
produce trap representations, or with unsigned types.

Of course this is just a bitwise version of (in the second case):

if (a!=b || b!=c || ... || z!=value)

except that the logical version can short-circuit, so fewer
operations are performed. (Though it's conceivable that on some
systems it'd be faster to perform all the bitwise operations than
to do the comparing and branching required to implement short-
circuiting.)

I don't think that would generally be as efficient as using multiple
comparisons against value because in that case the first mismatch will
prevent the rest from being evaluated.

More important, it's lousy code, and performance is rarely as
important as maintainability. And if performance *is* crucial in
this case, then it's probably time to redesign - it's unlikely that
keeping a lot of independent variables and testing them all against a
single value is actually the best way to accomplish whatever it is
that the problem requires.

That said, in some code I might find it defensible to write this as
a multiline controlling expression in an if statement (using the
logical operators, not the bitwise ones), as long as it was clear.

 

[Netocrat]

Or the similarly silly:

if (a^value|b^value|c^value|...|z^value)
/* one or more of a,b,c,...,z is not equal to value */

No that doesn't work. In the case of one variable with a==1, value==1
then a^value is false. I can't see a way to rearrange it using bitwise
or/and to work.

This really calls for a multi-way equality comparison:

if ( ! (a == b == c == d == ... == z == value) )

Obviously the semantics are wrong the way the equality operator is
currently defined, but I've sometimes wanted to use an expression like
this.

(Untested, but I think that's right.) Or similarly:

if (a^b|b^c|c^d|...|y^z|z^value)

Same problem as above.

All assuming that we're working with values that aren't going to produce
trap representations, or with unsigned types.

Right, these bitwise operations on signed integers are not portable.

Of course this is just a bitwise version of (in the second case):

if (a!=b || b!=c || ... || z!=value)

Except that the logical or can't be replaced with a bitwise or.

[...]

More important, it's lousy code,

Its purpose is not immediately clear and it depends on unsigned integers.
Both issues can be dealt with by appropriate commenting. More typical
expressions would avoid the need for commenting but I don't think that
makes it "lousy" code - it's perfectly serviceable, just a little atypical
and obscure.

and performance is rarely as
important as maintainability. And if performance *is* crucial in this
case, then it's probably time to redesign - it's unlikely that keeping a
lot of independent variables and testing them all against a single value
is actually the best way to accomplish whatever it is that the problem
requires.

That's quite a sweeping claim (as to likelihood of needing a multi-way
test) and I haven't given much thought to its merits but I'm curious by
what reasoning you make it.

[...]

 

[pete]

Except that the logical or can't be replaced with a bitwise or.

Logical or can be replaced with a bitwise or,
when the arguments are all boolean (either 1 or 0),
and it doesn't matter that there's no short circuiting.

(0 || 0 || 1 || 0) == (0 | 0 | 1 | 0)

 

[Netocrat]

Logical or can be replaced with a bitwise or,
when the arguments are all boolean (either 1 or 0),
and it doesn't matter that there's no short circuiting.

(0 || 0 || 1 || 0) == (0 | 0 | 1 | 0)

The statement quoted top of post was being compared with:

if (a^b|b^c|c^d|...|y^z|z^value)

They are equivalent statements only when each variable represents a single
bit. For a single bit, a ^ b is equivalent to a != b and as you pointed
out, a | b is equivalent to a || b. But for variables comprised of
multiple bits, the equivalence no longer holds.

So yes, I agree with you, but it doesn't apply in this case because a, b,
etc aren't boolean variables.

 

[Netocrat]

The statement quoted top of post was being compared with:

if (a^b|b^c|c^d|...|y^z|z^value)

They are equivalent statements only when each variable represents a single
bit. For a single bit, a ^ b is equivalent to a != b and as you pointed
out, a | b is equivalent to a || b. But for variables comprised of
multiple bits, the equivalence no longer holds.

So yes, I agree with you, but it doesn't apply in this case because a, b,
etc aren't boolean variables.

OK I've worded the concluding sentence inaccurately. Try this instead:
So your comment is valid and my original analysis should have rejected
the replacement of != with ^ rather than | with ||.

 

[Netocrat]

Please read the comment after the 'if' line.

It's identical to my original comment except for the addition of a couple
of commas. Please be more specific.

 

[Mark]

No that doesn't work. In the case of one variable with a==1, value==1
then a^value is false. I can't see a way to rearrange it using bitwise
or/and to work.

Albeit ugly, this should work:

if(a-b|b-c|c-d|d-e /* ... */)
puts("mismatched");
else
puts("pick one... they're all the same");

 

[Tim Rentsch]

No that doesn't work. In the case of one variable with a==1, value==1
then a^value is false.

Please read the comment after the 'if' line.